Touch sensitive electronic musical instrument responsive to only terminal velocities of keys



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ATTORNEYS United States Patent O Edward M. Jones, Cincinnati, Ohio, assignor to D. H. Baldwin Company, Cincinnati, Ohio, a corporation of Ohio Filed Dec. 30, 1966, Ser. No. 606,078 Int. "Cl. Gh 1/02 US. Cl. 841.01 17 Claims ABSTRACT OF THE DISCLOSURE An electronic piano in which a touch sensitive transient voltage is produced by a novel electromagnetic or capacitive transducer in response to striking of the key of a keyboard. This voltage is stored in a capacitor or capacitors via a diode or diodes, and the stored voltage allowed to decay according to the normal decay rates of vibration amplitude of a struck piano string. The decay is accelerated, i.e., damping occurs, on releasing the key, or a sustain pedal may disable the acceleration circuit. Double and triple rate decays may be employed. The decaying voltage is employed to gate through a tone, or to turn on an oscillator or to control a photoresistor modulator subjected to chopped light by a tone wheel.

The present invention relates generally to electronic musical instruments and more particularly to touch responsive devices and circuitry for producing piano tones in electronic musical instruments.

In the past, electrical keyboard instruments designed to simulate the musical performance of a piano have employed a wide variety of transducers for producing a control voltage proportional to the velocity of, or impact force applied to, each key. By use of such transducers it is possible to imitate the performance of a conventional vibrating string piano, at least to the extent that softness or loudness of the tone played depends upon speed at which the respective key is struck by the pianist, and hence upon key velocity. One of the earliest arrangementsby which piano tones were produced, at least partially through the use of electrical or electromagnetic circuitry, is disclosed in US. Patent No. 1,318,977 to Darley. The Darley instrument retains the srtings, which in a conventional piano vibrate in response to being struck by a hammer upon depression of an associated key; but each string is fabricated of magnetic material, and vibration thereof is effected by passage of a pulsating current through the coil of an electromagnet associated with each string. The magnitude of the pulsating current is a function of the amount of electrical resistance in the circuit to which the coil is connected, and this resistance is in turn dependent upon the speed with which the associated key is depressed. Since amplitude of vibration of a string is controlled by magnitude of the pulsating current, its current having a frequency identical to or a subharmonic of the natural frequency (fundamental) of vibration of the associated string, the velocity of a struck key governs the loudness of the tone. The Darley device, while certainly an innovation in its day, is relatively primitive when considering the present state of the art, requiring a rather complex and cumbersome action of mechanical parts to effect the desired resistance change. Nevertheless, the basic idea of a key operated transducer for conversion of kinetic energy, and more specifically velocity, of the key to an electrical control signal of proportional magnitude apppears to have its beginnings in the teachings of Darley.

Subsequent proposals relating to transducers of the general type mentioned above, suitable for use in electric 3,507,970 Patented Apr. 21, 1970 ICC pianos, have included capacitors whose discharge circuit time constant is controlled in accordance with the force with which the key is struck. In US. Patent No. 2,482,548, issued in 1949 to Kerhof, there is described a transducer in which a charged capacitor is discharged to a voltage level dependent upon the length of time a switch responsive to the striking of the key dwells upon a grounded contact in the path through which it travels. Another contact is disposed further along the path and provides an access point by which a control voltage may be supplied to a tone generator for productionof a desired tone therefrom. The intensity of the generated oscilla-' tions, i.e., the magnitude of the tone, is determined by the level of the control voltage obtained from the partially discharged capacitor as the switch armature resides against the further contact. The capacitor is then subjected to complete or substantially complete discharge through a pair of resistance circuits that determine the additive time constant of this subsequent discharge.

More recently, still other transducer arrangements have been proposed. One arrangement contemplates the production of a control voltage whose initial amplitude and rate of decay governs the passage and envelope of tones from a tone generator, the control voltage being impressed upon a capacitor in accordance with the intensity with which the key of the electronic keyboard instrument is struck. To this end, the transducer includes a perma nent magnet affixed to the end of the key opposite that exposed at the keyboard. Each key is mounted for rotation about a shaft such that when a key is struck the poles of the associated magnet are displaced from a rest position adjacent respective legs of a magnetic core on which a coil is wound. Since the voltage induced in the coil is a function of the time rate of change of flux therethrough as the magnetic poles are displaced, the magnitude of that voltage, which is applied to the aforementioned capacitor, depends upon the key velocity, (i.e., upon the intensity with which the key is struck). The amount of voltage stored on the capacitor determines the magnitude of a respective tone or tones corresponding to the note or notes associated with the struck keys. The discharge rate of, the capacitor is governed by the magnitude of the tones and the time constant of the circuit of which the capacitor is a component part.

In still another recent proposal, the key velocity-sensitive transducer includes a mechanism through which a hammer having a magnetic core mounted thereon is propelled toward a coil at a speed proportional to the force with which the associated key is struck. Entry of the core into the coil produces an increasing flux threading the coil and hence induces a voltage in the coil proportional to the time rate of change of that flux (and thereby, proportional to key velocity). This induced voltage is used to control generation of an appropriate tone. As will readily be observed, the latter proposal does not constitute a significant departure from the earlier proposals.

In general, the prior art transducers for electric pianos have been deficient in providing a faithful imitation of the conventional vibrating string piano, for a number of reasons. One substantial defect is that the tone produced when the key is struck builds up at a relatively slow pace, thus lacking the percussive effect produced by a piano. Another disadvantage of prior art electric pianos is that the touch responsive transducers have not been sufficiently isolated from one another; the striking of one key frequently brings on tones associated with other keys. Still another drawback lies in the failure to produce the desirable abrupt termination of a tone when the key is permitted to return to its rest position.

Accordingly, it is a primary object of the present invention to provide improvements in touch responsive transducers for operating percussion circuits in an electronic musical instrument.

According to the present invention a control signal is generated whose magnitude is in direct proportion to the velocity of a struck key. This is similar to the result achieved in the prior art as indicated above, but the manner in which the signal is generated differs significantly from the prior art and provides a number of advantages thereover. In particular, a capacitor, the voltage across which determines the loudness or softness of the tone played on the electronic keyboard instrument, is charged in accordance with the movement of a mechanism operatively associated with and so coupled for direct motion with the key that the physical parameters contributing to the existence of an associated force field are varied only as the key approaches the end of its travel. The extent to which the field is varied and the rate at which the variation occurs determine the voltage stored on the capacitor and the loudness (or softness) of the tone produced. The fact that the desired variation is controlled to occur only after the key has undergone substantial movement insures that there is a rapid tone start of the note associated with the struck key, corresponding to the fast buildup of tone achieved with a conventional hammer-actuated vibrating string.

In accordance with one embodiment of the invention, each key is adapted to actuate a respective piston, bellows, diaphragm or other mechanism with movable operating face, so as to increase the pressure on the face of another piston, bellows, diaphragm, or similar mechanism at the opposite end of an air chamber. The last-mentioned piston or similar mechanism forces or urges against a flexible resilient metal leaf constituting one plate of a capacitor so that the distance between the capacitor plates may thereby be varied. The buildup of appreciable pressure on the metal leaf is prevented until the key has undergone a major portion of its stroke, by provision of a decreasing clearance between piston and chamber walls as the piston proceeds into the chamber, or by use of a relief hole or relief valve through which air may escape from the chamber during the initial part of the stroke of the member producing the pressure on the metal leaf.

The change in spacing of the plates of this pneumatic capacitor varies the capacitance (inversely with distance between plates) and, of course, any force field existing between the plates. In effect, the impedance of the capacitor is proportional to the velocity of the key and controls the voltage to which a further capacitor is charged, the further capacitor having the pneumatic capacitor in its charging circuit. The voltage across the further capacitor is utilized to control the magnitude of the output tone.

In accordance with another embodiment of the invention, each key actuates, through an appropriate linkage, a separate magnet or pole piece thereof positioned for movement within an encompassing core on which a coil is wound. The end terminals of the coil are connected to the plates of a capacitor to be charged, through a diode. The movable pole piece is positioned within an air gap of the core and is arranged to be struck by a linkage fastened to the key only if the key has undergone a substantial portion of its travel. In this manner a rapid trans fer of flux is effected from that portion of the core in which the magnet. is housed to the core portion about which the coil is wound; hence, a pulse having a relatively rapid rise time is generated at the output terminals of the coil to charge the capacitor. If the voltage of the capacitor is utilized to control the generation of tone at the output of the instrument, it will be observed that the tone amplitude is proportional to the maximum key velocity since the size of the pulse generated by the coil is proportional to maximum key velocity. This is the relationship existing in an actual percussive instrument having a relatively hard hammer coupled to the playing key.

Thus, this embodiment of electric piano also provides a faithful imitation of a conventional vibrating string piano.

It is therefore a further object of the present invention to provide an electrical keyboard instrument in which the striking of the key is closely followed by the charging of a capacitor in accordance with the movement of a mechanism operatively associated and coupled for motion with the key so as to vary the physical parameters contributing to the existence of an associated force field to such an extent and at such a rate that the loudness or softness of the output tone of the instrument is accurately determined.

Another object of the present invention is to provide an electrical musical instrument of the keyboard type, in accordance with the above-mentioned objects, wherein rapid start of tone is achieved upon striking of the key to faithfully initiate a conventional vibrating string piano.

Still another object of the present invention is to provide a keyboard instrument of the aforementioned type wherein tones produced by the striking of the key are controlled by an electrical or electromagnetic arrangement wherein desired variations in a force field are achieved only after the key has undergone a substantial portion of its travel.

A further object of the present invention is to provide an electric piano in which each key actuates a piston, bellows, diaphragm or other mechanism with movable operating faces, to increase the pressure in an air chamber at the other end of which is located a resilient membrane by which forces are transmitted to a metal leaf com-prising one plate of a movable plate capacitor, the final capacitance of which is dependent upon key velocity and determines the envelope of tones generated by the instrument.

It is yet another object of the present invention to provide a piano in which the generation of tones is accomplished electromagnetically, each key controlling the transfer of flux through a coil to which a capacitor to be charged is connected so that voltage pulses induced in the coil are effective to charge the capacitor and thereby to control the envelope of the output tones.

The above and still further objects, features and attendant advantages of the present invention will become apparent from a consideration of the following detailed description of certain preferred embodiments thereof, especially when taken in conjunction with the accompanying drawings, in which:

' FIGURE 1 is a side view of key and associated mechanical components and of the variable capacitance mechanism of one embodiment of the invention, including a circuit diagram, partly schematic and partly block diagrammatic, of the associated network;

FIGURE 2 is a sectional view illustrating the manner in which the variable plate capacitor is operated with depression of the key;

FIGURE 3 is a circuit diagram of a network for controlling rate of decay of tone components;

FIGURE 4 is a side view and associated circuit diagram of another embodiment of key operated transducer according to the invention;

FIGURES 5 and 6 are sectional views illustrating still further embodiments of the invention, and

FIGURES 7-11 are block diagrams of variants of the system of FIGURE 1.

Referring now to FIGURE 1, a key 10 on the keyboard of the electronic musical instrument is arranged to pivot, in the same manner as each of the other keys, on a pivot rail 12 as the front or free end of the key is depressed by the player. Normally, the key has a rest position with its uppermost surface residing adjacent a felt pad 15 on an up stop rail 16, and upon striking, the key moves to a position adjacent or in contact with a felt pad 18 on the down stop rail 19. Upon release of the key it is returned to its normal rest position under the force exerted by a return spring or any other conventional device suitable for that purpose.

Pivotally coupled to the key for vertical motion therewith each time the key is struck and released is a rod 27 on which a piston is mounted. Piston 25, which may alternatively be a bellows, diaphragm, membrane, or other force-actuable mechanism by which to build up air pressure within associated chamber 30, moves into the chamber when key 10 is struck and withdraws from the chamber as the key returns to its rest or normal position, as shown. Chamber 30 may be in the form of a cylinder or hole of other suitable shape, depending upon the shape of piston 25, within a metal bar 32 which is electrically grounded.

The other end of chamber 30 is closed by a rubber diaphragm 35 extending along the underside of metal bar 32 (as viewed in FIGURE 1) and suitably fastened thereto. Residing against the surface of the diaphragm 35 remote from bar 32 is a thin resilient metal leaf 36 which is normally maintained at a preselected distance from a further metal strip 41 by an insulating block or bushing 37 and an insulating strip 40. Metal strip 41 is secured in insulating plate 43 which is secured to metal bar 32 by a screw 45 extending through plate 43, insulating strip 40, bushing 37 and into a tapped counterbored hole in bar 32.

The underside of key 10 is also provided with a plunger 28 adapted, when the key is depressed, to force a contact wire or metal strip 66 from a grounded metal electrode 72 retained within a rotatable rod 70. Metal contact strip 66 is retained at a predetermined distance from the key by a mounting block 69 of insulating material. Rod 70 is rotated by the sustain pedal (not shown) of the keyboard musical instrument.

Metal strips 36 and 41 form the plates of a pneumatic or variable plate capacitor 42 whose capacitance varies inversely with the distance between the plates. Strip 41 is connected to one end of an inductor having a grounded center tap and inductively coupled to the output winding of an RF oscillator 53. The other end or terminal of inductor 50 is connected through a variable capacitor to metal leaf or strip 36 of pneumatic capacitor 42 and to agrounded load resistor 56. The voltage across the load resistor 56 is applied to the anode of diode 58, the cathode of which is connected to a capacitor 60 to be charged. The voltage on capacitor 60 controls the magnitude of tones generated, in accordance with the striking of the key 10. A resistor 63 is connected in parallel with capacitor 60, and a further resistor 64 is connected from the high side of the parallel combination of capacitor 60 and resistor 63 to resilient metal contact wire 66. The voltage across capacitor 60 may be monitored to control the generation or passage of tone for the note associated with key 10. It will be understood that each key of the keyboard instrument is provided with an arrangement corresponding to that which has been thus far described.

In one arrangement the voltage across the capacitor may be applied to one input terminal of a push-pull modulated photoresistor 75, acting as a gate, and which are illuminated from a tone wheel 77, in known manner. The gate provides for amplitude of tone generation which is dependent on magnitude of voltage at the first input terminal. This permits variable amplitude gating of a tone to an amplifier 78 and thence to loudspeaker 80. Tone is delivered to amplifier 78 as the gate circuit is activated in response to depression of a key at the keyboard of the musical instrument.

In operation of the embodiment of FIGURE 1, striking of key 10 is accompanied by movement of piston 25 into air chamber 30. However, air pressure in chamber 30 does not build up appreciably until key 10 is closed to the lowermost point of its path, by virtue of the varying outer diameter of piston 25. That is, initially, the clearance between the piston wall and the cylinder wall is quite large so that air escapes in large quantity as the piston enters the cylinder. This situation continues to exist until the largest diameter portion of piston 25 begins to enter the cylinder or air chamber 30 at which point a very small clearance is present between the piston wall and the cylinder wall. Hence, continued movement of the piston into the chamber is accompanied by pressure on the portion of rubber diaphragm 35 adjacent the other end of th chamber.

As entry of piston 25 into chamber 30 progresses, the air pressure on diaphragm 35 causes the latter to be distorted outwardly, and thus exert a force on resilient metal leaf 36 of pneumatic capacitor 42. This situation is illustrated more clearly in FIGURE 2 where the distortion of diaphragm 35 is of such magnitude as to force resilient metal leaf 36 against insulated strip 40. The inertia of metal leaf 36 is preferably made as small as possible by use of a very thin leaf and by placement of its fastening point a great distance from the point or points at which forces are exerted by rubber diaphragm 35 as it is distorted. By making the inertia of the movable plate 36 extremely small, its displacement depends, for practical purposes, only on the instantaneous air pressure within chamber 30 (and exerted on diaphragm 35). The peak air pressure within the chamber is dependent on and proportional to the velocity of key 10 which is in turn dependent upon the impact force applied to the key as it is struck by the player of the keyboard instrument. The greater the velocity of the key, the greater is the amount of pressure built up within chamber 30, and hence the greater is the capacitance value of pneumatic capacitor 42. Thus a higher signal is applied to load resistor 56 from the paths including capacitors 42 and 56, and inductor 50 coupled to oscillator 53, with increasing key velocity. The

effect is the charging of capacitor 60 to a higher voltage through diode 58 and thus an increase in the amplitude of the output tone produced as the force with which the key is struck, and the key velocity, is increased.

By preventing the buildup of air pressure or substantial air pressure in chamber 30 until the key has approached the end of its movement after being struck, the output tone for the note associated with the struck key starts quickly, taking only a few milliseconds to reach its maximum value, rather than building up at a relatively slow rate. The rapid production of tone in response to striking of a key is, of course, characteristic of a conventional piano.

As previously explained, the latter result is achieved in the embodiment of FIGURES 1 and 2 by use of the variable diameter piston 25. However, other arrangements such as a relief hole in metal bar 32 may be utilized in the case of other pressurizing devices such as bellows or diaphragms or the like.

Moreover, the existence of an air leak, either by means of a small relief hole or the controlled clearance shown in FIGURES 1 and 2, renders the movement of the capacitor plate 36 only momentary. That is, plate 36 moves downwardly for an instant sufiicient to permit capacitor 60 to be charged to a voltage having a value proportional to the peak capacitance attained by pneumatic capacitor 42. Immediately after undergoing the movement in the direction of plate 41, plate 36 returns to its normal position since the air pressure causing the outward distortion of diaphragm 35 leaks oflF-rapidly from chamber 30. Thus, there is no generation of tone as key 10 returns to its normal or rest position after being struck.

Variable capacitor 55 is a neutralizing capacitor, and is adjusted in value to provide zero signal to load resistor 56 when pneumatic capacitor 42 has the capacitance value existing with capacitor plate 36 in the rest position. As plate 36 approaches plate 41 a differential voltage is applied to resistor 56, indicative of an unbalance in impedance between the two paths for signal from the RF oscillator 53. Thin insulating strip 40 may be composed of mica to permit a high capacitance value for pneumatic capacitor 42.

Immediately after capacitor 60 has been charged at the peak value of voltage permitted by the capacitance value of capacitor 42, capacitor 60 begins to discharge through resistor 63. As key returns to its normal position, plunger 28 mounted at the bottom thereof withdraws from metal contact wire 66 so that the latter is permitted to contact grounded electrode 72, thus placing resistor 64 in parallel with resistor 63 in the discharge circuit for capacitor 60. This places a smaller value of resistance in the discharge circuit than the value of either of resistors 63 or 64 so that capacitor 60 discharges quite rapidly and the tone for the note associated with the struck key dies quickly. Sustaining of the tone is accomplished by depression of the sustain pedal of the instrument which in turn rotates rod 70 so that electrode 72 is moved out of contact with metal wire 66. Thereby, the rapid damping occasioned by the placement of resistor 64 in the capacitor discharge circuit is prevented as a result of the removal of the ground connection from resistor 64.

The voltage stored on capacitor 60 may be utilized to control the transmission or amplification of the tone by application to the gate 75, composed of photocells 75b and transformer 75a. Gate circuit 75 may be looked at as of a type which permits passage of tone from tone wheel 77 upon application of a voltage from capacitor 60 and reduces the amplitude of the passing tone in accordance with the decrease in voltage on the capacitor, so that when the capacitor 60 has completely discharged, the modulated light no longer causes any AC voltage in the transformer 75a, and the gate is effectively closed (nonconductive).

Broadly, the voltage stored on the capacitor may be applied to a transducer or sensor for converting energy modulations of any type to modulations of an electrical parameter. For example, light falling on photocell circuit 75b, to which is applied voltage from capacitor 60 may be modulated by rotating disk 77 in the path between the light source and the photocell, in accordance with the tone oscillations to be produced. Hence, the output signal of the photocell is also modulated and when the photocell supply voltage decreases the magnitude of the output modulated voltage is also decreased until the output tone decays to an imperceptible volume. Any of these arrangements, as well as others which will be described in conjunction with FIGURES 7-11 of the drawings, are suflicient, in conjunction with the details of the invention as heretofore described, to permit faithful reproduction and imitation of the conventional vibrating string piano.

It is desirable, in order to produce a better imitation of actual percussion instruments, such as the conventional piano, in which the string is struck by a hard hammer when a key is depressed, that various components of the tone decay at different rates. A circuit for this purpose is shown in FIGURE 3, wherein components corresponding to those in FIGURE 1 are designated by like reference numerals.

In FIGURE 3, a coil 50 having a grounded center tap and inductively coupled to the output winding of RF oscillator '53 applies equal but oppositely phased signal to each of the paths containing pneumatic capacitor 42 and neutralizing capacitor 55. The output resulting from the instantaneous difference in the capacitances of capacitors 42 and 55 is supplied to load resistor 56 and operates to charge each of parallel coupled capacitors 60, 102, 107 through its respective diode 58, 100 and 105. The time constants for the discharge circuits of the three capacitors 60, 102, and 107 may be made to differ by appropriate adjustment of capacitor values and the values of the respective resistors 63, 103 and 109 in their discharge aths. p A plunger attached to the key, similar to the arrangement shown for the sustain circuit in FIGURE 1, may be utilized to open switch 115 while a key is in struck position and to clase switch 115, whereby the capacitors 61, 102 and 107 are discharged in accordance with the value of resistor 112, when the key returns to its normal rest position.

The node between the charge capacitor and its respective diode, such as junction 121 in FIGURE 3, may be connected to one of several different photo-resistive transducers PCl, PCZ, PC3, equal in number to the number of capacitors to be charged via pneumatic capacitor 42, thus to permit modulation of output signal from the respective transducer. The tone outputs of the transducers, are subject to modulated light deriving from the same or different tone tracks, for the several transducers. That is, broadly conceived, the voltage on each capacitor may be applied to control a separate and distinct tone source or an individual gate connected to a tone generator, so that separate frequencies having different rates of decay may be applied to the audio portion of the circuit, or so that we frequently may have plural rates of decay.

FIGURE 4 illustrates another arrangement of the pneumatic capacitor and of the neutralizing capacitor in circuit therewith. Except as otherwise shown and described, the embodiment of FIGURE 4, both in mechanical and electrical features, may be identical to that illustrated in FIGURE 1. As key 10 approaches the felt pad 18 on down-stop rail 19, its underside encounters a rubber tube 132, preferably of rectangular cross section and closed at both ends. Tube 132 has a relief opening therein to permit air to escape as the tube is depressed by the key. A metal leaf 130 is mounted on the underside of the tube 132 and is connected to one end terminal of grounded center tap winding 50. Another metal leaf or printed circuit 133, insulated by a layer 133a on its upper surface, and spaced from leaf 130 by insulative spacer 137, is mounted under tube 132 and is connected directly to a load. A further metal strip is utilized in conjunction with metal leaf 133 as neutralizing capacitor 55 (FIGURE 3), strip 140 being separated from leaf 133 by insulating strip 138. Strip 140 is also supplied with a finger 142 downwardly extending therefrom to permit adjustment of the capacitance value of neutralizing capacitor 55 to equal that of pneumatic capacitor 42 when rubber tube 132 is in its normal or rest position.

It is to be observed that key 10 does not squash rubber tube 132 until the key has reached a point close to the end of its path. This arrangement is similar to that of the controlled clearance provided between the piston wall and cylinder wall in the arrangement of FIGURE 1, to ensure rapid starting of tone.

Impact on rubber tube 132 by key 10, as the latter approaches the end of its path of travel after having been struck, changes the relative separation between plates 130 and 133 of pneumatic capacitor 42, and hence varies the capacitance and impedance of that path with respect to the capacitance and impedance of the path containing the neutralizing capacitor 55.

Operation of the remaining circuitry may be identical to that described above with respect to FIGURE 1. The embodiment of FIGURE 4, however, has the advantage that the leafs 130 for several notes may simply be thin metallic fingers extending from a single thin metal strip, and that the leaf 140 for neutralizing capacitor 55 may be a single strip having several fingers corresponding to the different notes, each comprising a portion 142 which may be bent in accordance with the desired adjustment of each note.

In each of the embodiments shown and described above RF oscillator 53 may be replaced by a DC source, and the stationary electrode or plate and thin insulator of the pneumatic capacitor replaced by a resistance element such as evaporated metal, sprayed carbon, solid carbon, or conductive plastic or rubber. Such a configuration is in effect a pneumatic resistor.

FIGURES 5' and 6 illustrate electromagnetic arrangements for charging the capacitor from which the control voltage for the output tone is obtained.

Referring to FIGURE 5, a square-D shaped soft iron core 200 is provided with a small air gap 203 into which an L shaped pole piece 205 of a magnet 207 extends. Magnet 207 is mounted on a leaf spring 210 which is fastened to the core by rivet 212 or other suitable fastening device.

The coil 215 is wound about a coil form 217 on the lower portion of core 200 as viewed in the figure. The ends of the coil connected to opposite plates of the capacity 230 to be charged, one terminal of the coil connected to the capacitor plate by a diode 232 and the other terminal connected to a point of reference potential, ground in this case.

A plunger 235 is fastened to the underside of the key (not shown) and is spaced from pole piece 205 so that contact therewith occurs only after the key has undergone a substantial portion of its travel after being struck by the player of the instrument. When pole piece 205 is struck by plunger 235, it is displaced from a position adjacent one end of gap 203 to a position adjacent the other end and thereby effects a change in magnetic flux through coil 125. In consequence, a positive pulse is generated during the downward motion of the key to charge capacitor 230 through diode 232. The negative pulse which is generated during the return of the key upwardly to its rest or normal position cannot, however, pass through the diode because of the manner in which the latter is poled. Hence no tone is generated during the return motion of the key.

The size (amplitude) of the generated pulse is proportional to the maximum key velocity, and controls the magnitude of the voltage stored on condenser 230 by which the tone amplitude is controlled. This is the same relation that exists in any actual percussive instrument, as in the conventional piano where a relatively hard hammer is coupled to the playing key for impact with a string when the key is struck.

Since the flux through magnet 207 and pole piece 205 remains relatively constant in every position thereof, there is relatively little interference with key movement from the, magnetic force. Moreover, the configuration of the electromagnetic system, with the magnet encompassed within soft iron core, provides excellent magnetic shielding between adjacent notes.

Soft paper or plastic pieces 239 and 240 may be ap-' plied to the ends of core 200 adjacent gap 203 in order to limit the travel of the armature (L-shaped pole piece 205) so that there is no large unbalance of magnetic forces between or at extreme positions of the armature.

As the struck key returns to its normal position the plunger 235 withdraws from hole 242 within core 200, and armature 205 returns to its uppermost position under the action of leaf spring 210.

In the embodiment of FIGURE 6, the soft iron core 250 is G-shaped and has a coil 252 wound about form 253 at its lower extremity. Each core 250 is mounted directly below a respective key of the keyboard instrument and is protected by a felt pad 18 at its upper portion tocushion the load exerted by the key when the latter is struck by the player.

A magnet 255 is mounted within air gap 256 in confronting relation with soft paper or plastic piece 257. In the embodiment of FIGURE 6 the soft iron core structure 250 must be sufficiently stiff to resist the magnetic attraction across the gap so as to prevent subsequent locking when the gap is closed, but must be sufficiently resilient or elastic to close the gap in response to a hard blow from key 10.

As in the embodiment of FIGURE 5, the amount of flux traversing the foil 252 changes in accordance with the width of air gap 256, the width of the gap being a function of the impact force and hence the velocity of key 10. Hence the pulse generated by the coil is proportional to the velocity of the key.

In FIGURES 7-11, 10 is a key of a keyboard musical instrument, which actuates a touch sensitive generator 300. Suitable touch sensitive generators are illustrated in FIGURES 1, 2, 4, 5, 6 of the accompanying drawings, but any device or circuit may be used which generates a voltage proportional to or which is an appropriate direct function of the velocity with which a key is struck.

In FIGURE 7, the touch responsive voltage is stored via a diode 301 in a parallel RC circuit, having resistance R1 and capacitor C1. The voltage of capacitor C1 decays through R1, on termination of the applied voltage, and is utilized to render conductive a gate 302, which gates through the output of a tone source 303 to have an envelope shape like that of the capacitor voltage shape. The output of the gate is applied to an amplifier 304 and loudspeaker 305.

A parallel discharge path exists for capacitor C1 via resistance R2. This resistance proceeds to ground via contacts 306, so long as key 10 is not actuated. When the key 10 is actuated, arm 307 is depressed by the dotted line coupling 308 to key 10, separating contacts 306 and breaking the circuit to ground. On release of the key, arm 307 rises to bring the contacts 306 together, which again completes a circuit to ground for resistance R2, and rapidly discharges capacitor C1. R1 is thus very large, to provide a time constant of one half to one second, but inclusion of R2 in the discharge path provides damping, i.e., rapid discharge. On operation of a sustain pedal 310, rod 311 is rotated, separating the contacts 306, even when the key is not depressed. This pedal is called a damping pedal in the piano but in fact has the function of preventing damping.

In operation, when key 10 is depressed, a touch representative voltage is transferred to capacitor C1. So long as the key 10 is held depressed, the voltage on C1 decays slowly. When the key 10 is released, decay occurs rapidly, unless sustain pedal 310 is actuated. If it is, the slow decay obtains regardless of Whether or not the key 10 is released.

In FIGURE 8 the voltage of C1 energizes or biases on a self-excited tone oscillator 312, in substitution of the gate 302 and continuously-operative tone source 303, In other respects, the systems of FIGURES 7 and 8 operate identically.

The system of FIGURE 9 includes provision for multiple decay, i.e., on generation of a pulse b'y generator 300, capacitors C1, C3 and C4 are all identically charged, via diodes 301, 320 and 321. Time constants are arranged so that R1CI R3C3 R4C4. Each discharge proceeds independently of the others, since the diode 320 isolates C3 from C1, because C1 is, after the pulse, always at smaller voltage than is C3, but the diode 320 is poled to conduct positive current from C1 to C3 and not from C3 to C1. The voltages across C1, C3, C4 are additively mixed in mixer 322, to form a complex triple-decay-rate gating wave, which is applied to gate 302. p

The use of double or triple-rate decay circuits more nearly simulates the actual decay of a piano tone than does a single-rate decay. RlCl can be selected to produce a relatively rapid decay rate, so that the total output of the mixer 322 is reduced rapidly to a value near that set by the sum of the voltages on C3 and C4. Thereafter, the rate decays slowly until the sole voltage remaining at appreciable level is that due to C4. C4R4 may require 1-10 seconds to decay to a gate-off level. R2 serves to discharge all the capacitors rapidly, since once C4 is discharged to below C3, C3 dumps into C4, and similarly for C1.

FIGURE 11 uses the gate RC circuitry of FIGURE 9, but the voltages across C1, C3, C4 may independently gate three tones, originating in sources 330, 331, 332 via gates 333, 334, 335 to a linear tone mixer 336, and thence to amplifier 304 and speaker 305. The oscillators or tone sources 330, 331, 332 may be adjacent in frequency, may have the same fundamental frequency but different harmonic structures or may be at widely-different, unrelated frequencies as would be present in a bell. So, the tones may differ by less than 1%, or they may be harmonically-related, or related as the partial of a chime or bell, etc. The system of FIGURE 11 is generic to the system of FIGURE 3, in that the same triple ratedecay, touch-sensitive, gating wave generators are illustrated in both figures, but are applied to photo-cell gates in FIGURE 3, of the type specifically illustrated in FIG- URE 1. Certain piano tones are produced by striking three strings simultaneously. These strings are intercoupled, so that each string does not decay at its own rate,.as if the other strings were absent, but the strings have each a complex rate of decay established by its own rate and the fact that it is transferring energy to and acquiring energy from other strings, slightly detuned with respect to its own frequency. The systems of FIGURES 3 and 11 enable three string vibration and decay to be simulated. The tone generators of FIGURES 7, 9, 10, 11 and the associated gates may accordingly be taken to represent either tone wheels or light choppers plus photoresistors, or tone oscillators plus diode gates.

In FIGURE 10, as soon as the capacitors C1, C2, C3 are charged, which occurs in /2 to 5 milliseconds at the last part of the downward motion of the key they start discharging through R1, R2, R3, respectively. The rates of decay are unequal, i.e., if C1=C2=C3, then Rl R2 R3. As each capacitor discharges through its own parallel resistance, it also tends to spill charge over into any capacitor which tends to discharge faster than it discharges. The net voltage at terminal 340 is then the sum of three capacitor voltages, which may be the result of three decays, i.e., fast, intermediate and slow, say 30 ms., 100 ms. and 250 ms.

While I have disclosed certain preferred embodiments of my invention, it will be apparent that variations in the specific details of construction which have been illustrated and described may be resorted to without departing from the spirit and scope of the invention, as defined in the appended claims.

I claim:

1. In an electronic keyboard instrument for producing tones in accordance with the striking of keys, the combination comprising means comprising a variable impedance for establishing a variable force field,

means responsive to striking of a key for producing over the major travel of said key in response to said striking a substantially Zero variation of said field and only in response to a minor travel of said key occurring substantially at the end of such travel a sudden transient impulsive variation of said field having an amplitude in accordance with only the terminal velocity of said key,

circuit means responsive to said variation of said field for generating a control impulse of corresponding amplitude, and

means responsive to said control impulse amplitude for controlling the magnitude and wave shape of a tone produced by said instrument in response to the striking of said key.

2. The combination according to claim 1 wherein said means establishing said field comprises a capacitor having a pair of plates, at least one of said plates being movable relative to the other.

3. The combination according to claim 2 wherein said means for establishing a variable force field includes a resilient membrane in contact with said movable plate,

a pressure chamber having said membrane as one wall thereof,

a linkage including means for increasing the pressure in said chamber in response to striking of the respectively associated key,

said chamber including means for substantially preventing an increase in pressure therein until said key has undergone said substantial movement,

said means for reestablishing the original field including a pressure leak in said chamber.

4. The combination according to claim 2 wherein said means for establishing a variable force field includes a resilient insulating container of air having a pair of end Walls,

said plates mounted on respective exterior surfaces of said end Walls,

said container positioned for crushing thereof in delayed response to striking of said key for varying the distance between said plates,

said means for reestablishing the original field comprising a hole in said container.

5. In an electronic keyboard instrument for producing tones in accordance with the striking of keys, the combination for each of a plurality of keys of,

means establishing a variable force field,

means for varying the magnitude of said field in delayed response to the striking of an associated key and in accordance with the velocity of the struck y,

circuit means responsive to the varying magnitude of said force field for generating a control voltage proportional thereto,

means responsive to the control voltage for controlling the magnitude of the respective tone produced 'by the instrument in accordance with the amplitude of said control voltage,

wherein said means for varying said field includes means for reestablishing the original field after only momentary variation thereof,

wherein said means for varying the magnitude of said field comprises a linkage coupled to said key for movement therewith, and means operatively associated with said means establishing a variable force field and separated from said linkage for actuation to vary said field in response to movement of said linkage only after said key has undergone movement through a substantial portion of its path of travel, and wherein said means establishing said field comprises a capacitor having a pair of plates, at least one of said plates being movable relative to the other.

6. The combination according to claim 5 wherein said means operatively associated with said means establishing a variable field includes a resilient membrane in contact with said movable plate, a pressure chamber having said membrane as one wall thereof, I

said linkage including means for increasing the pressure in said chamber in response to striking of the respectively associated key,

said chamber including means for substantially preventing an increase in pressure therein until said key has undergone said substantial movement,

said means for reestablishing the original field including a pressure leak in said chamber.

7. The combination according to claim 5 wherein said means operatively associated with said means establishing a variable field includes a resilient insulating container of air having a pair of end walls,

said plates mounted on respective exterior surfaces of said end walls,

said container positioned for crushing thereof in delayed response to striking of said key for varying the distance between said plates,

said means for reestablishing the original field comprising a hole in said container.

8. In an electrical musical instrument simulating a piano,

signal storage means,

variable impedance means coupled to said signal storage means,

means for varying the impedance of said variable impedance means in proportion to and during only the terminal travel of a struck key of said instrument to the exclusion of the major initial part of said travel, including means for enabling said impedance varying means only after said struck key has moved through a predetermined initial portion of its path of travel,

means for supplying a unidirectional impulsive signal to said signal storage means in amplitude proportional to an impedance variation of said variable impedance means,

tone generating means, and

means responsive to signal stored by said signal storage means for enabling passage of tone in a predetermined wave shape having a maximum determined by the magnitude of said signal.

9. In a touch sensitive electrical musical instrument,

a key having a travel in a path between a fully up and a fully down position,

first electromechanical means responsive to motion of said key only during the termination of said travel for providing an impulsive voltage having an amplitude which is a direct function of the velocity of said key during said termination of said travel, said first means including a storage capacitor means,

diode means interposed between said first means and said storage capacitor means,

said diode means being poled and connected to charge said storage capacitor means in response to saidimpulsive voltage and to prevent discharge of said storage capacitor means via said diode means,

resistive timing discharge means connected to said storage capacitor means for enforcing sustained discharge of said storage capacitor means following said impulsive voltage, and

means for generating a tone signal having an initial amplitude corresponding with the amplitude of said momentary voltage and a decay envelope corresponding generally with the voltage decay curve of said storage capacitor means through said resistive timing discharge means.

10. In a touch sensitive electrical musical instrument,

a key having an up and a down position,

first means responsive to actuation of said key to said down position for providing a momentary voltage having an amplitude which is a direct function of the touch with which said key is actuated from its up to its down position,

a storage capacitor means,

diode means interposed between said first means and said storage capacitor means,

said diode means being poled to enable charge of said storage capacitor means in response to said momentary voltage and to prevent discharge of said storage capacitor means via said diode means,

resistive timing discharge means connected to said storage capacitor means for enforcing timed discharge of said storage capacitor means,

means for generating a tone signal having an initial amplitude corresponding with the amplitude of said momentary voltage and a decay envelope corresponding generally with the voltage decay curve of said storage capacitor means, and wherein said resistive means includes photoresistive means, and wherein said last mentioned means includes means for illuminating said photo-resistive means with light of intensity modulated at tone frequency. 11. The combination according to claim 10 wherein said last mentioned means includes a tone generator, a gate in cascade with said tone generator, a load in cascade with said gate, and means applying the voltage of said storage capacitor means as a gating voltage. 12. The combination according to claim 10, wherein is provided means for imparting to said decay envelope a double rate of decay, said double rate of decay having an initial relatively rapid decay and a terminal relatively slow decay, said decays being selected to simulate the decay of a struck piano string. 13. The combination according to claim 10 wherein is provided means responsive to return of said key to said up position after depression to said down position for providing an accelerated discharge of said storage capacitor means. 14. The combination according to claim 13 wherein is provided a damping pedal, and

means responsive to actuation of said damping pedal prevents accelerated discharge of said storage capacitor means. 15. An electronic musical instrument comprising multiple keys, means including a velocity sensitive electro-mechanical transducer separately responsive to each keyfor directly transducing the velocity of its action when struck into an impulsive unidirectional electrical signal proportional to and occurring only during the terminal part of the downward motion of said key to the exclusion of the part of said motion preceding and following said terminal part, means for generating sustained tone signals in response to said impulsive electrical signal, and means for acoustically transducing and radiating said sustained tone signals. 16. The combination according to claim 15, wherein said transducer is a pneumatically operated capacitor.

17. The combination according to claim 15, wherein said transducer is an electromagnetic transducer.

References Cited UNITED STATES PATENTS 2,296,125 9/ 1942 Traub 841.27 2,570,178 10/1951 Zuck 84-l.ll 2,623,996 12/1952 Gray 841.ll 3,002,411 10/1961 Potzl 84l.0l 3,248,470 4/ 1966 Markowitz et a1. 84l.01

HERMAN KARL 'SAALBACH, Primary Examiner S. CHATMON, 1a., Assistant Examiner Us. 01. X.R. 841.26 

